1. Field of the Invention
The present invention generally relates to the field of electronics, and in particular to electrical oscillators and methods and means for tuning such oscillators; particularly in electrical watch applications.
2. Description of the Prior Art
The accuracy required in electronic watches and similar circuits, coupled with the small size and durability which must be maintained, dictate and use of crystal controlled oscillators. However, these oscillators must also maintain a very high precision. As used here, "accuracy" means the ability to maintain the initial oscillation rate, while "precision" means the preciseness of the original frequency setting.
Maintaining adequate accuracy is no problem with crystal-controlled oscillators, but the untuned precision of crystals is not adequate for electronic watches and in similar applications. It has proven necessary to tune such oscillators by the addition of variable amounts of parallel or series reactance. Generally, the added reactance has been capacitive, and must be adjusted after it is added to the oscillator circuit manually. Conversely, the necessary adjustment has been made by the addition of an electrically variable reactance such as a varactor, but the controlling electrical signal which varies the varactor reactance must be permanently maintained and consequently requires a manual adjustment of the controlling electrical signal. Although ways of adjusting the controlling electrical signal electronically are possible, they would be and are too cumbersome and expensive to be useful. For crystal oscillators which are to be used in electrical watches using analog displays and using digital readouts, the constraints placed on size and cost have ruled out any form of tuning to obtain the precision required except by manual adjustment in the prior art. A further disadvantage of the prior art is that the reactance elements used to tune such oscillators have been susceptible to tuning drift and have, therefore, impaired the accuracy of the crystal oscillator. This has made compromises between accuracy and precision necessary.
Thus a need exists for a tuning means which can be machine implemented with less time and labor consumption required and which is less subject to frequency drift.